Switch to pytest as a test runner

[jmadler]

This change switches from 'python setup.py test' to 'pytest' as a test runner under the tox environment, and encourages use of tox.

This eliminates a class of test failures under Python 3 caused by usage of the unsupported path of combining tox and 'python setup.py tests':

https://tox.readthedocs.io/en/latest/example/basic.html#integration-with-setup-py-test-command

Specifically in our case, the PYTHONPATH included . and src/, which resulted in e.g. reprlib being discovered under src/ instead of in the core stdlib.

[jmadler]

From Travis CI for py27:

python setup.py test:

Ran 1086 tests in 43.175s
OK (skipped=26, expected failures=62)

pytest:

======= 1002 passed, 26 skipped, 62 xfailed, 1 warnings in 44.67 seconds =======

The additional 4 tests are likely from the set skipped on travis previously by discover_tests.sh.

This doesn't fix all of the Python3 test failures, but does bring Py34 green.